Pyrolytic graphite waveguide utilizing the anisotropic electrical conductivity properties of pyrolytic graphite



Aug. 4, 1970 I DAVID J. uuv 3,

'PYRQLYTIC GRAPHITE WAVEGUIDE -UTILJIZING THE ANISOTROPIC ELECTRICALCONDUCTlVlTY PROPERTIES OF PYROLYTIC GRAPHITE Filed Jan. 2, 1969 2Sheets-Sheet 1 33 I ll Mom-z /l/NPUT FILTER OUTPLT 3. Illll 32 INVENTOR.DAVID J. LIU

Fig. 3 WM ATTORNEY v, g- 4, 1970 DAVED -J. LIU 3,522,561

PYROLYTIC GRAPHITE WAVEGUIDE UTILIZING THE ANISOTHOPIC ELECTRICALCONDUCTIVITY PROPERTIES OF PYHOLYTIC GRAPHITE Filed Jan. 2, 1969TRANSMISION INVENTOR.

DAVID J. LIU BY 2 Sheets-Sheet 2 v United States Patent US. Cl. 333-95 6Claims ABSTRACT OF THE DISCLOSURE A section of waveguide fabricated frompyrolytic graphite material suppresses all modes except the transverse Hmode thereby permitting large distant transmission of millimeter Wavesby eliminating the parasitic effects of unwanted modes.

Communications systems of ever increasing channel capacity are ingreater demand; accordingly, efiicient transmission at millimeter wavesis very attractive. The present invention provides a system foreflicient transmis sion at these frequencies.

Prior art dielectric filled waveguides have not been very satisfactoryat millimeter wave lengths due to the required dielectric materialsbeing dissipative. At millimeter wave lengths overmoded waveguides arenot easily avoided and overmoded waveguides produce additionalattenuation from dielectric materials which is an order of magnitudelarger than the attenuation due to the metal walls.

Guides with anisotropic surface impedance provide better performancethan other types of waveguides. Among these are disc, corrugated, andhelical waveguides. Disc and corrugated waveguides provide the ease withwhich the designer can choose, and realize, the required surfaceimpedance. However, some technological difiiculties are encountered inmanufacturing tolerances and sharp edges on the disc, and so forth.

Helical Waveguides overcome ditficulties associated with the productionof disc waveguides yet retain the same benefits. The pyrolytic graphitehollow circular Waveguide contemplated by the present invention providesa more efficient anisotropic surface impedance than any of the prior artwaveguide mentioned above.

Pyrolytic graphite is formed by passing hydrocarbon gas over a hotsurface held at about 4000 F. The carbon atoms are removed from the gasby a thermo-decomposition process and are deposited in a manner similarto vacuum plating operations. The material deposits such that it isalways a poor conductor perpendicular to the deposition surface and agood conductor parallel to the deposition surface. A sheet of pyrolyticgraphite thus formed consists of many a layer of carbon atoms. In thedirection perpendicular to the deposition surface very weak bondingforces hold these atomic layers together. This results in a dielectricconstant of five times that of vacuum and slightly larger than thedielectric constant of structural ceramic. A companion result is thatthe mechanical strength of the material is weak in a directionperpendicular to the deposition layer, tensile strength of about 500 to1000 p.s.i. being normal. This means that the material is mechanicallyeasy to work in this particular direction. While in the directionparallel to the deposition surface the electric conductivity is at least40% greater than copper and the dielectric constant is about the sameorder of magnitude as that of a good conductor.

A circular waveguide of pyrolytic graphite made in such a way that thetransverse cross sectional plane being the deposition surface and a holeof proper size drilled through perpendicular to the deposition surfacecan cer- 3,522,561 Patented Aug. 4, 1970 tainly support the circularelectric modes better than other modes. The attenuation suffered bythe-lowest circular electric mode in a pyrolytic graphite circularhollow waveguide is only about 71.5% of that of a uniform copper pipe.On the other hand, the attenuation suffered by the circular magneticmode and other hybrid modes (including electric modes of higher order)in a pyrolytic graphite waveguide is about 26 db greater than a uniformcopper pipe. The realization of 26 db additional attenuation for highermodes is not possible to achieve by means of prior art waveguideincluding the iris, dielectric, disc loaded and helical waveguidesmentioned earlier. The inventor has discovered that this characteristicfeature of pyrolytic graphite waveguides is of importance for millimeterwave transmission over very long distances. It is contemplating inpracticing the present invention that a small section of waveguide willbe made of pyrolytic graphite material according to the principles ofthe present invention. Thereafter the Waveguides can be constructed ofany suitable material, perhaps of a metal such as copper and so forth.The idea being that the small section of waveguide fabricated inaccordance with the present invention will act as a mode filtersuppressing all modes except the one desired, namely the H (or the TEmode.

It is well known that there is a continuous interaction of the variousmodes; accordingly, if a wave is launched at one end of a waveguide andis multi-moded, as will be the case in all prior art devices, theseother modes will constantly inter-react with the wanted mode and willincrease dissipation for every mode and will cause separate eddycurrents to be circulated resulting in energy loss. If all modes but thedesired mode are suppressed right from the beginning by the mode filteraction provided by the present invention, the millimeter wave can travelgreat distances retaining much of its initial power.

Therefore an object of the present invention is to provide a wave devicefor operation at millimeter wave frequencies.

Another object of the present invention is to provide a millimeterwaveguide fabricated from pyrolytic graphite.

Another object of the present invention is to provide an efficient modefilter.

Another object of the present invention is to provide a section ofwaveguide which suppresses all modes but the TEm mode.

Another object of the present invention is to providean antenna forlaunching millimeter waves suppressing all modes except the TE mode.

Other objects, features and advantages of the present invention will bebetter understood from the following specifications when read inconjunction with the attached drawings of which:

FIG. 1 is a section of pyrolytic graphite waveguide.

FIG. 2 is a mode filter.

FIG. 3 is a millimeter wave antenna.

FIG. 4 shows one end of a waveguide with coaxial connection.

FIG. 5 is an end view of a waveguide.

FIG. 6 is a side view of the waveguide shown in FIG. 5.

Referring to FIG. 1 we see a waveguide which has been fabricated inaccordance with the principles of the present invention. The waveguideconsists of several sections. Between sections joint 24 is seen. Eachsection is /2" thick and is cast separately. A waveguide is made up ofseveral separate sections. Each section being about /2" thick, to make awaveguide 4 long requires 8 sections bonded together. The bondingmaterial needed can be any of a number of strong adhesives for exampleArmstrong C7 epoxy which are well known in the prior art. Each sectionis cast in the desired shape by passing a hydrocarbon gas over a surfaceof the die-shape that is held at approximately 4000 F. The carbon atomsare removed from the gas by thermal-decomposition and deposited in amanner similar to vacuum plating. The material is deposited such that itbecomes an insulator perpendicular to the deposition surface and a goodconductor parallel to the deposition surface. Accordingly a mandrile canbe inserted on the central portion of the required surface to providethe desired wave guide opening, with regard to the ends, openings forbolts 22 four separate smaller mandriles can be used. The outside shapeis provided by the shape of the die-like shape of the required surfacewherein the gas is passed over it to form the waveguide. Each section ofwaveguide is shown with lines running through it indicating thedirection of the layers. 35 indicates these layers run in the requireddirection.

Once an appropriate number of sections are bonded together with theflanges at each end making up a waveguide, the internal surface is to besanded and polished until smooth and clean. 21 shows the Waveguideopening.

In FIG. 2 a section of waveguide is shown ideally with layers 35 makingup mode filter 33. Input 31 is launched into the waveguide and emergingtherefrom is output 32. The input will be a multimoded wave having asubstantial number of modes. However only TE mode will emerge fromwaveguidethe parasitic modes are suppressed.

The internal dimensions of a waveguide fabricated for use at 100 gHz.has a dimension of 1 inside diameter and a 2" outside dimension. Theflange dimensions are approximately 3 /2" square and providingsatisfactory bolt holes in its four corners. Obviously at 1" insidediameter a 100 gHz. signal would be considerably overmoded in thiswaveguide.

If it is desired to launch this wave into the air, the ends can beflared as shown in FIG. 3 with an appropriate angle 29 of approximately45 from the outside edges. Each section is cast in a similar manner tothe preceding sections but with mandrile to provide the desired angularshape. The individual sections will then be bonded together to make theantenna as shown in FIG. 3.

If the wave is to travel a great distance the section made of pyrolyticgraphite need only be small and probably only occupy the first portionof waveguide. Thereafter the waveguide would be made of metal which iswell known in the present state of the art. In any event the waveguidewill have an input applied as shown in FIG. 4, coaxial cable 41 having aflange 42 mates with the flange section of waveguide 35. Bolts 38 whichextend through both flange elements providing the means for launching awave into the waveguide which provides mode filtering. Probe 43 is anyof the well known configurations which are state of the art.

Referring to FIG. 5 we see the end view of a section of waveguidefabricated in accordance with the present invention. We also see thetransverse H mode. This mode will be discussed further as we proceedwith the details of the mathematical relationship consistent with thedescription of the present invention. We see that the magnetic field ofthe H mode is radial While the electric field is normal thereto.Referring to FIG. 6 we see the side view of that section of transmissionwaveguide shown in FIG. 5. Transmission is in accordance with thedirection indicated in 44. Walls 21 are polished walls within thewaveguide. We note layers 16 are normal to the direction oftransmission. These layers are the same as that which the direction ofdeposition is accomplished.

In order to fully understand the relationship of the pyrolytic graphitein its interaction with the various modes to suppress some whilepermitting certain others to pass without significant attenuation, thepyrolytic graphite region is examined mathematically in the followingparagraphs.

In the pyrolytic graphite region, Maxwells equations are:

typical frequency of gHz. in the millimeter waves range, one has:

and

(G) a. 2.03(10s =7D32 103 In other words a permittivity tensor of thepyrolytic graphite may be formed as follows:

E; 0' 0 0 al Therefore, in the millimeter waves range, the followingpermittivity tensor is valid.

a, 0 0 6 (l-j Q) One can solve Equations 1 through 6 for E E H and H interms of E and H as follows:

Introducing the notation A =A one derives from Equation 12:

To solve Equations 13 through 17, let

and one obtains, in general and so on. To solve Equations 18 and 19, let

cr ='l 'l' z z and one obtains, in general and for H mode one obtains:

6.4(' f n vl- (mm nep ers/ meter where a is the radius of the waveguidein meters, and f the operating frequency, and

For E-modes one obtains the attenuation factor aEv also by perturbationmethod resulting at a typical frequency in the millimeter waves range ofgHz.:

(26) db/ kilometer Thus we see in Equation 28 that the attenuationfactor of the H mode is significantly small in comparison with theattenuation factor of the E modes. Accordingly the TE mode asrepresented by the attenuation factor of the H mode will result in awave guide which permits the TE mode to pass with minimal attenuationwhile all other modes are completely dissipated within a few Wavelengths of waveguide. Accordingly unlike prior art waveguides where theinteraction of the different order modes produces a significantattenuation due to the interaction of these modes with one another suchthat higher order modes become parasitic. Each have dissipationassociated with their own mode which is eliminated by the presentinvention.

Accordingly with the present invention millimeter waves can be launchedat one end of a mode filter according to the invention and emerge tocontinue any distance through conventional waveguide having only onedesired mode which experiences minimal attenuation thereafter. It isapparent then with the present invention Wave guide systems formillimeter waves can be increased in efliciency by merely the insertionof a section of wave guide in accordance with the principles of thepresent invention which acts as a mode filter. Furthermore the waveguidecan be shaped as to permit mode filtering and launching of a wavesimultaneously in the form of an antenna. My invention has beendescribed with reference to specific apparatus and many of those skilledin the art will be able to make many substitutions and variations in theabove described apparatus without departing from the true scope andspirit of the present invention. Accordingly, the inventor wishes onlyto be limited in his invention by the appended claims.

I claim:

1. A pyrolytic graphite wave device having,

a plurality of layers of pyrolytic graphite,

a smooth polished circular opening perpendicular said layers ofpreselected dimentions, and

flange means for attaching a probe to launch a wave therein.

2. A pyrolytic graphite Wave device according to claim 1 wherein saidopening flares out at a predetermined angle at the end opposite saidflange.

3. A pyrolytic graphite wave device according to claim 2 wherein saidpredetermined angle is 45.

4. A pyrolytic graphite wave device according to claim 1 which furtherincludes second flange means for connecting said wave device to sectionsof metallic wave guide thereby acting as a mode filter.

5. A pyrolytic wave device according to claim 4 where in said opening isone inch in diameter.

6. A pyrolytic wave device according to claim 4 where in said pluralityof layers make up a number of segments, said segments bonded together toform a continuous wave guide.

References Cited UNITED STATES PATENTS 3,016,502 l/l962 Unger. 3,158,82411/1964 Larsen et -al 333-98 X (Other references on following page) 7FOREIGN PATENTS 8/1958 Germany. 5/1962 Germany.

OTHER REFERENCES 5 General Electric, Pyrolytic Graphite Eng. Hndbk.

8 its and Boron Alloys of Pyrolytic Graphite, Chem. and Physics ofCarbon, vol. 2, pp. 225-256, Marcel Dekker Inc., New York, 1966.

HERMAN KARL SAALBACH, Primary Examiner W. H. PUNTER, Assistant ExaminerUS. Cl. X.R.

